Decorative thermoplastics have been widely used as protective coverings from flooring to electronics. Thermoplastics can be melted and molded, making them suitable for virtually any application where a protective covering is desired.
There are various processes commonly used to inlay a decoration onto thermoplastics. These processes, such as in-mold labeling (IML), film insert molding (FIM) and in-mold decoration (IMD), involve printing a film with a decorative graphic; inserting the film into the injection mold; and injecting a thermoplastic molding compound behind the film. When the film is removed from the injection mold, the printed film has theoretically adhered to the molding compound and acts as the decorative first surface. The printing process itself typically involves methods such as silk-screening, rotary screening, flexographic printing techniques and offset lithography printing, among others.
In practice, the combination of the printing processes and inlay processes described above can result in a graphic that is not durable and can be easily damaged, abraded or chemically removed from the printed surface of the finished film. In order to avoid such potential damage, standard practice has been to print the backside of a transparent film, and then place the film in the mold with the graphic facing the inside of the mold. Molding compound is then shot against the graphically printed and coated surface. This yields a graphic that is protected by a layer of transparent plastic film on the first surface.
However, printed graphics cannot easily survive the conditions inside the injection mold, which can be up to 12,000 psi and 600 f. Graphics are burned and sheared off the surface of the films that are facing the inside of the mold, when the hot resins flow in through the injection gates. Methods have been developed for specialized gating that will allow specific areas within the mold to support film insert molding without destroying the graphics, as long as specialized and specific processing parameters are maintained.
Another attempt to address the problem of graphic survival inside the mold is to coat the graphic with a high temperature coating that will protect the graphic from the hot molding compounds that are injected into the mold and onto the graphically printed surface that has been coated. However, the coatings used to protect the graphics cannot create a miscible bond with the hot flowing molding compounds made up of thermoplastic resins. In fact these protective coatings generally interfere with and defeat a quality lamination bond between the decorated films and the hot molding compounds. Therefore, specialized adhesion promoters have been developed that will help to improve the bond between the protective coatings on the in-mold decorative films and the hot flowing resins. A series of adhesion promoters have been developed specific to each of the thermoplastic resins and protective coatings used in molding the product. However, delamination between the decorative film and the molded parts remains a challenge.
A further obstacle for printed thermoplastics is that the printed and coated films are often thermoformed to conform to the shape of the mold before they are inserted into the injection mold. Therefore the film printing must also be able to survive the heating and stretching of the film over profiles and complex curves during the thermoforming process. Inks that can go through a vacuum forming process, “Formable inks”, have been developed. However, because these formable ink systems are pigment-based inks and printed to the surface of the films, they can only withstand film stretching of less than half inch before the graphic print thins and degrades. This failure severely limits the scope of form factors that can be decorated using the traditional in-mold decoration methods in practice today.
There exists a need in the art for a variably decorated thermoplastic film which retains its graphic throughout the molding process, as well as throughout the use of the film to which it is applied.